1. Field of the Invention
The present invention relates to a color liquid crystal display (LCD) device adapted for high density display in colors, and also to a method for driving such a display device.
2. Discussion of the Background
It is noticeable of late that the industrial attention is concentrated on color LCD devices of a constitution having light shielding layers and color filters on the inner surface thereof.
One example of the above is known as an active matrix LCD device where an active matrix element such as TFT (Thin Film Transistor)or MIM (Metal-Insulator-Metal) used practically in a liquid crystal television receiver and so forth is provided for each of the pixels.
In such active matrix LCD devices, each pixel is driven in a static or similar mode, so that the requirement for gap control of the liquid crystal layer is not exactly strict.
However, it is difficult to realize a great dimensional increase of such active matrix LCD device suited generally for use in a small-sized portable television receiver or the like. Accordingly, there have been some difficulties in attaining adequate application to any large-sized personal computer, word processor and so forth where high-density display characteristics are requisite.
Meanwhile, there may exist some cases of employing, instead of the active matrix device used customarily heretofore, a twisted nematic (TN) type dot matrix LCD device which is driven in a dynamic mode.
Since the TN type dot matrix LCD device is so constituted that an active matrix element is not provided for each of the pixels, it becomes possible to manufacture a large-sized high density display device with facility, whereas a problem arises with regard to deterioration of the display quality.
For example, in a display device with a capacity of 640.times.400 dots employed generally in a personal computer or the like, the driving duty factor reaches 1/200 or so to eventually bring about some disadvantages that the viewing angle is extremely narrow and the contrast is rendered very low. Consequently, there are achievable merely eight colors at most with R-G-B in color display, and it has been substantially impossible in practical use to realize a display with gray scales.
Recently a super-twisted nematic (STN) type LCD device has been developed as means for realizing a high density dot matrix display by the technique of increasing the twist angle of liquid crystal molecules between the two electrodes to induce a steep voltage-transmission curve.
According to this technique, however, the value of .DELTA.n.d, which is product of the birefringence .DELTA.n of the liquid crystal and the thickness d of the liquid crystal layer in the LCD device, is substantially in a range of 0.8 to 1.2 microns, and a high contrast is obtainable merely in the combination of specific hues alone such as yellowish green and dark blue, bluish purple and light yellow, and so forth.
Since such a LCD device is not suited for monochromatic or black-and-white display as mentioned, there exists a disadvantage that multi-color or full-color display is impossible in combination with a micro color filter.
Meanwhile, there is proposed an improved technique as disclosed in EP 246842, wherein an inverse-twisted liquid crystal cell or a retardation film are laminated to constitute a retardation compensator on such a super-twisted liquid crystal cell for dot display, and the elliptical polarization caused in the dot-display liquid crystal cell is compensated by the inverse-twisted liquid crystal cell to eliminate undesired coloring peculiar to the STN type LCD device, thereby achieving a display similar to the monochromatic one.
Due to employment of such a retardation compensator, there is obtainable a desired display substantially equal to a monochromatic one, hence raising a possibility of color display by the use of a color filter in combination with such a display device.
In particular, by combining the above color filter with a light shielding layer and disposing them in a cell, it becomes possible to realize a color LCD device apparently having a high contrast without any positional deviation.
However, in the STN type LCD device which utilizes birefringence with the liquid crystal cell having such a twist angle, even a slight variation in the thickness of the liquid crytal layer is visually represented as color due to the birefringence so that, unless the thickness of the liquid crystal layer corresponding to an inter-substrate gap is retained to be remarkably uniform, there occurs nonuniformity of the background color to consequently blur the display with conspicuous deterioration of the display quality.
Therefore, it has been required to develop an improved color LCD device which is capable of ensuring high display quality while maintaining sufficient uniformity with facility despite any large area of the inter-substrate gap.